The waxing and waning of a bloom of pyrosomes, Pyrosoma atlanticum, in the southern California Current System, 2014-2019

We analyzed the recent outbreak of the pyrosome, Pyrosoma atlanticum, from plankton samples collected in the southern sector of the California Current System (CCS) by CalCOFI and the CCE-LTER program. Numerous reports of this free-swimming pelagic tunicate, including far north as the Pacific Northwest, inspired a detailed spatial and temporal analysis of this organism’s population in the southern CCS. We first analyzed Zooscanned images of CCE-LTER PRPOOS net tow samples from 2005-2019 to identify the onset of the population outbreak. These results showed that 2014 was the onset of this bloom in the southern California region, which led to detailed spatial analysis of CalCOFI bongo net samples from spring cruises 2014-2019 to infer sources of introduction into the study region. Additionally, we assigned life history stage (larvae: juvenile) to infer population growth characteristics. Results suggest that hydrographic conditions in the CCS beginning in spring 2014 provided conditions conducive to introduction of pyrosomes, possibly from the west and/or north, which subsequently showed elevated C biomass in spring 2015, slightly diminished biomass in spring 2016, a maximum in 2017 mostly represented by tetrazooid larvae, and a population decline in 2018. The potential impacts of pyrosomes on pelagic and benthic ecosystems in this region have yet to be fully explored.